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From Jeremy Hanna <jeremy.hanna1...@gmail.com>
Subject Re: Massive writes when only reading from Cassandra
Date Mon, 17 Oct 2011 21:22:37 GMT
Even after disabling hinted handoff and setting read_repair_chance to 0 on all our column families,
we were still experiencing massive writes.  Apparently the read_repair_chance is completely
ignored at any CL higher than CL.ONE.  So we were doing CL.QUORUM on reads and writes and
seeing massive writes still.  It was because of the background read repairs being done.  We
did extensive logging and checking and that's all it could be as no mutations were coming
in via thrift to those column families.

In any case, just wanted to give some follow-up here as it's been an inexplicable rock in
our backpack and hopefully clears up where that setting is actually used.  I'll update the
storage configuration wiki to include that caveat as well.

On Sep 10, 2011, at 5:14 PM, Jeremy Hanna wrote:

> Thanks for the insights.  I may first try disabling hinted handoff for one run of our
data pipeline and see if it exhibits the same behavior.  Will post back if I see anything
enlightening there.
> 
> On Sep 10, 2011, at 5:04 PM, Chris Goffinet wrote:
> 
>> You could tail the commit log with `strings` to see what keys are being inserted.
>> 
>> On Sat, Sep 10, 2011 at 2:24 PM, Jonathan Ellis <jbellis@gmail.com> wrote:
>> Two possibilities:
>> 
>> 1) Hinted handoff (this will show up in the logs on the sending
>> machine, on the receiving one it will just look like any other write)
>> 
>> 2) You have something doing writes that you're not aware of, I guess
>> you could track that down using wireshark to see where the write
>> messages are coming from
>> 
>> On Sat, Sep 10, 2011 at 3:56 PM, Jeremy Hanna
>> <jeremy.hanna1234@gmail.com> wrote:
>>> Oh and we're running 0.8.4 and the RF is 3.
>>> 
>>> On Sep 10, 2011, at 3:49 PM, Jeremy Hanna wrote:
>>> 
>>>> In addition, the mutation stage and the read stage are backed up like:
>>>> 
>>>> Pool Name                    Active   Pending   Blocked
>>>> ReadStage                        32       773         0
>>>> RequestResponseStage              0         0         0
>>>> ReadRepairStage                   0         0         0
>>>> MutationStage                   158    525918         0
>>>> ReplicateOnWriteStage             0         0         0
>>>> GossipStage                       0         0         0
>>>> AntiEntropyStage                  0         0         0
>>>> MigrationStage                    0         0         0
>>>> StreamStage                       0         0         0
>>>> MemtablePostFlusher               1         5         0
>>>> FILEUTILS-DELETE-POOL             0         0         0
>>>> FlushWriter                       2         5         0
>>>> MiscStage                         0         0         0
>>>> FlushSorter                       0         0         0
>>>> InternalResponseStage             0         0         0
>>>> HintedHandoff                     0         0         0
>>>> CompactionManager               n/a        29
>>>> MessagingService                n/a      0,34
>>>> 
>>>> On Sep 10, 2011, at 3:38 PM, Jeremy Hanna wrote:
>>>> 
>>>>> We are experiencing massive writes to column families when only doing
reads from Cassandra.  A set of 5 hadoop jobs are reading from Cassandra and then writing
out to hdfs.  That is the only thing operating on the cluster.  We are reading at CL.QUORUM
with hadoop and have written with CL.QUORUM.  Read repair chance is set to 0.0 on all column
families.  However, in the logs, I'm seeing flush after flush of memtables and compactions
taking place.  Is there something else that would be writing based on the above description?
>>>>> 
>>>>> Jeremy
>>>> 
>>> 
>>> 
>> 
>> 
>> 
>> --
>> Jonathan Ellis
>> Project Chair, Apache Cassandra
>> co-founder of DataStax, the source for professional Cassandra support
>> http://www.datastax.com
>> 
> 


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